LP 1772993: Ability to blank Age Hold Protection in Vol/Copy Editor.

LP 1772993: Ability to blank Age Hold Protection in Vol/Copy Editor.

Add the <NONE> option to the age-based hold protection field in the
web staff client Vol/Copy Editor.

To test:

1. Add a value for age-based hold protection to a copy.

2. Attempt to remove the value in the copy editor.  Note that the
   dropdown does not allow you to do so.

3. Apply this commit.

4. Repeat step 2.  Now there is a <NONE> option that removes the value
   from the copy.

Signed-off-by: Jason Stephenson <jason@sigio.com>
Signed-off-by: Jeff Davis <jdavis@sitka.bclibraries.ca>